2 Answers
2

What you need a minyan for is mentioned in the Mishnah Megilah 4(3); the link is to the Blackman edition with English translation.
The paragraphs in the Wikipedia article entitled "Rituals requiring a minyan" and "Prayer with a minyan" seem to be reliable.

Shulchan Aruch 90:9: Try to daven in shul. If he is "annus" and can't make it at least daven "together" with the congregation (at the same time). And if is "annus" and couldn't make it on time to the minyan at least daven by himself in the shul.
Mishan Brura (28) explains the power of the davening together with a congregation: Hashem doesn't "moes" (dislikes?) the davening of a congregation.